Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common cause thus far identified for both familial and idiopathic Parkinson's disease (PD). The majority of LRRK2 linked cases present clinical symptoms typical of idiopathic or sporadic PD. Therefore, understanding of mechanisms underlying LRRK-induced neurodegeneration will likely provide new insights into pathogenesis of PD and open new avenues for therapeutic intervention. Compelling evidence suggest that mitochondrial dysfunction could represent a critical event in the pathogenesis of PD. However, the mechanisms underlying mitochondrial dysfunction in PD remain elusive. Given that several lines of evidence suggested the mitochondrial presence of LRRK2, a possible mechanism of LRRK2 action is at mitochondria. Mitochondria are dynamic organelles that undergo continual fission and fusion events which serve crucial physiological function and are regulated by a machinery involving large dynamin- related GTPase that exert opposing effects. These mitochondria fission and fusion proteins control not only mitochondrial number and morphology but also mitochondrial distribution and function. Notably, an altered mitochondrial dynamics plays a role in mitochondrial dysfunction in cells treated with PD-related toxins and in cells expressing pathogenic mutant PINK-1, Parkin or DJ-1, suggesting that an altered mitochondrial dynamics may be a common pathway leading to mitochondrial dysfunction during PD pathogenesis. More recent studies demonstrated a potential functional interaction between LRRK2 and PINK1/Parkin suggesting that LRRK2 could also be involved in this pathway. Indeed, in our preliminary studies, we found that overexpression of LRRK2 (G2019S) mutant causes mitochondrial fragmentation and abnormal distribution and reduced expression of DLP1/OPA1 along with mitochondrial dysfunction in SHSY-5Y human neuroblastoma cells and primary rat cortical neurons. Based on these observations, our overall hypothesis is that LRRK2 mutations cause abnormal mitochondrial dynamics which in turn causes mitochondrial ultrastructural defects, dysfunction and redistribution which adversely affects neuronal function including causing synaptic abnormalities in PD. To address this hypothesis, the following two specific aims will be pursued. Aim 1 To determine whether mutant LRRK2 cause abnormal mitochondrial fission/fusion and dysfunction in neurons;Aim 2: To determine how mutant LRRK2 affects mitochondrial fission/fusion. PUBLIC HEALTH RELEVANCE: It is of paramount significance to understand how LRRK2 mutations cause familial Parkinson disease. Based on recent development in the field and our preliminary results, we propose to investigate whether LRRK2 mutants cause mitochondrial abnormalities via its toxic effect on the balance of mitochondrial fission and fusion. The completion of this project will enable us to collect data for a more in-depth mechanistic study that may help identify novel therapeutic targets